The 1969 Federal Coal Mine Health and Safety Act requires "each operator of a coal mine to take accurate samples of the amount of respirable dust in the mine atmosphere to which each miner in the active working place of such mine is exposed." The amount of respirable dust referred to in the Act is "the average concentration of respirable dust if measured with an MRE instrument or such equivalent concentrations if measured with another device approved by the Secretary and the Secretary of Health, Education, and Welfare." Part 74 of Title 30, Code of Federal Regulations, sets forth the requirements which must be met in order to be an approved alternate sampling device.
At the present time there are several personal respirable dust samplers, i.e., samplers which can be carried by the miners, which have been approved as alternate devices for sampling coal mine environments. Each approved device consists of a pump unit, a sampling head assembly, and, if rechargeable batteries are used in the pump unit, a battery charger.
The sampling head assembly, the construction of which is essentially the same for all of the approved alternate units, consists of a 10 mm cyclone separator, and a compatible filter assembly. The function of the cyclone separator is to separate the non-respirable fraction of the particulate contaminants in the sampled aerosol from the respirable fraction. The respirable dust, i.e., that fraction which penetrates through the cyclone, is collected by the filter assembly.
Although the present cyclone-filter configuration of the sampling head assembly has been accepted and is in widespread use throughout the coal mining industry, it suffers several disadvantages. At a mass respirable dust concentration of 2.0 milligrams per cubic meter of air, which is the mandatory maximum standard for underground and surface coal mines, the total sample mass collected during a full working shift is often less than one milligram. The analytical precision error for a mass of this magnitude can be as high as twenty percent. Further, the cyclone separator requires daily cleaning and maintenance. Proper assembly and alignment of the cyclone and filter stages requires several o-ring seals between assembly stages to prevent air leaks. Faulty o-rings are difficult to detect and can result in excessive sampling error.